A mathematical model for (K ion)o clearance, incorporating both passive diffusion and active uptake processes, was developed. After specifying the initial conditions (the spatial profiles of (K ion)o at time equals 0 following electrical stimulation), the experimental data were incorporated into the mathematical formulation. Passive diffusion propbably makes a relatively insignificant contribution to the clearance of elevated (K ion)o in the cortex. Active processes are probably more important in this regard.